Owing to the known interaction between photons and electrons, great advances have been made in the field of photodetectors in recent years, particularly in those photodetectors that utilize semiconductor materials. One type of semiconductor-based photodetector is termed an avalanche photodiode. This type of structure is generally composed of a number of solid semiconductive materials that serve different purposes such as absorption and multiplication.
The avalanche photodiode structure provides the primary benefit of large gain through the action of excited charge carriers that produce large numbers of electron-hole pairs in the multiplication layer. However, an avalanche photodiode is so efficient at producing large numbers of charge carriers that it runs the risk of becoming saturated, thus adversely affecting the bandwidth of the device. In order to prevent charge carrier breakdown, it is imperative that the electric field be regulated within the avalanche photodiode itself, and in particular it is desirable to have the electric field in the multiplication layer be significantly higher than that in the absorption layer.
Mesa avalanche photodiodes have exposed high field p-n junction that are difficult to passivate using a layer of insulating material. Therefore, standard commercial InP/InGaAs avalanche photodiodes use planar diffused structures which bury the p-n junction, leading to robust, long lifetime devices. However, these InP avalanche photodiodes require extremely accurate diffusion control of both the depth and the doping density of the p-type semiconductor regions. This critical control is essential, since this diffusion controls the magnitude of the electric field in the multiplication region, the length of the avalanche region, as well as the charge control.
There are no truly planar InAlAs avalanche photodiodes that have been proposed. An existing avalanche photodiode has an etched isolation ring which is etched down to expose the top of the high field avalanche region followed by a deep titanium implant to further isolate the high field region. This is then followed by a zinc diffusion to contact the p-type semiconductor region. This is a very complicated structure requiring critical etching and implant steps. In spite of these efforts, it is believed that the lifetime of this avalanche photodiode is ten times shorter than their standard planar avalanche photodiode and thus not sufficient for telecommunications use.
Therefore, there is a need in the art for a simple yet effective avalanche photodiode that is capable of being produced cheaply and with efficient critical control. Accordingly, the present invention comprises a planar avalanche photodiode including a first n-type semiconductor layer defining a planar contact area, and a second n-type semiconductor layer having a p-type diffusion region. Further features of the structure include an n-type semiconductor multiplication layer and an n-type semiconductor absorption layer, and a p-type contact layer. The p-type diffusion region is disposed directly adjacent to the p-type contact layer, thereby decreasing the capacitance of the planar avalanche photodiode while increasing the speed, lifetime, and cost-effectiveness of the structure. Further embodiments and advantages of the present invention are discussed below with reference to the figures.